Various Codes
Successfully complete the game under the easy or normal difficulty settings to view the first ending and unlock the "Mission Mode", "Gallery", "Ghost List" options and the "Feel" and "Expand" camera abilities. Additionally, a new game can be
started with the current items, maps, and camera abilities.

Successfully complete the game under the normal difficulty setting to also unlock the "Hard Mode" options, two alternate costumes (red/blue kimono and dark blue kimono), and glasses for Mio.

Successfully complete the game under the hard difficulty setting to view the alternate ending and unlock the "Nightmare Mode" and "Album" options, the true ending sequence, and two more alternate costumes (short kimono and Yae and
Sae kimono).

Successfully complete the game under the nightmare difficulty setting to view the alternate ending and to unlock two more alternate costumes (leather).

Successfully complete all missions in mission mode to unlock the maid costume and glasses for Mayu.

Successfully complete all missions in mission mode with an "S" rank to unlock two additional costumes.

Successfully complete the Ghost List to unlock set-up information.

Unlock Bonuses
Follow the criteria listed to unlock the corresponding features:
Beat game on Easy/Normal: Unlock Rei List, Mission Mode, Gallery and the Feel and Expand Camera. You can also start a game with your weapons/items from the finished game.
Beat game on Normal: Unlock Hard Mode and two bonus costumes, plus accessories.
Beat game on Hard: Unlock Nigftmare Mode, plu Album feature. You'll also unlock more costumes and see the final ending.
Beat game on Nightmare: Unlock two new costumes.
Beat all missions in Mission Mode: Unlock more new costumes.